Transmission power control circuits are installed in radio communication devices to maintain a constant transmission power. A transmission power control circuit typically controls such that the transmission output level is kept constant by detecting part of the transmission power by a wave detector, and adjusting the gain of an amplification circuit or an attenuator by feedback of the wave detection voltage. Such transmission power control is also disclosed in Patent Document 1, for example.
A diode used as a wave detector for detecting radio waves in the tens of GHz can only accept wave detection voltages with a dynamic range of approximately 20 to 30 dB. FIG. 7 is a diagram showing wave detection characteristics in a 38 GHz band used for fixed access communication and the like. In FIG. 7, if the range in which accurate wave detection is possible is 10 mV to 1V, the dynamic range D is limited to 26 dB.
As described above, the dynamic range in which power control is possible is often limited by a wave detector with a high frequency band. Therefore, it is difficult to meet requirements for wide transmission power ranges such as dynamic ranges of 50 dB or 80 dB. As proposals to solve such a problem, there are techniques disclosed in Patent Document 2 and Patent Document 3 for example. A transmission output control circuit disclosed in Patent Document 2 includes a wave detection circuit, a calculating amplifier, a calculating amplifier, and an automatic output control circuit. The wave detection circuit receives part of an output of a power amplification circuit, which amplifies a transmission signal. The calculating amplifier receives the output of the wave detection circuit, and can switch the gain in a plurality of steps. The automatic output control circuit receives the output of the calculating amplifier. In this transmission output control circuit, the output of the automatic output control circuit is fed back to the power amplification circuit, and the transmission output is controlled in a plurality of steps.
A transmission power control circuit disclosed in Patent Document 3 includes a signal branching unit, a switching unit, and a control unit. The signal branching unit, on the output side of a distributor for distributing part of the output of the amplifier, inputs a high harmonic signal, branches it to a plurality of high harmonic signals, attenuates them such that the branched plurality of high harmonic signals become high harmonic signals whose levels vary in steps at fixed intervals, and outputs them. The switching unit inputs the plurality of high harmonic signals output from the signal branching unit, selects one of the high harmonic signals among the input plurality of high harmonic signals according to an instruction of a switching control signal, and outputs it as a transmission output signal. The control unit outputs the switching control signal. In the transmission power control circuit, a switching control signal is output from the control unit, and a control voltage is output to change the gain of the amplifier corresponding to the switching operation of the output signal by the switching unit.